This invention relates to a method and an apparatus for the spectroscopic measurement of concentrations of several components in a gas mixture. The gas mixture has at least one component A which has an absorption line spectrum and whose concentration in the gas mixture exceeds the concentrations of other components to such an extent that the measurement of the concentration of such other components is interfered with. Conventionally, a measuring cuvette is filled with the gas mixture and is irradiated by a continuous-spectrum light beam emitted by a light source. The light beam emanating from the measuring cuvette is guided through a spectrometer for spectral dispersion and is thereafter sensed by a detector. The concentration of the other components of the gas mixture is determined from the intensity of the absorption lines of the recorded light.
Thus, a light beam emanating from a light source is guided through a measuring cuvette which contains the gases to be examined. The light is absorbed by the components differently for different wavelengths, so that by measuring the light intensity before and after the passage of the light through the cuvette, the transmissions are determined for several wavelengths and the concentrations are determined by Lambert-Beer's law. The utilized wavelength range extends from the ultraviolet to the infrared wavelengths.
Such measuring instruments which are designated as spectral photometers, spectral spectrometers or array spectrometers are utilized in laboratory analyses and in the process measuring technology. These instruments permit a continuous measurement of gases, for example, for monitoring ammonia, sulfur dioxide and nitrogen oxides in the fumes of a fossil fuel power station or for an optimal control of systems for the removal of nitrogen.
Apparatus which include instruments of the above-outlined type have been marketed by numerous firms for laboratory use. Also, apparatus usable in manufacturing processes are obtainable. Some of these utilize ultraviolet lamps with a continuous emission spectrum, such as deuterium and xenon lamps.
Thus, German Offenlegungsschrift (application published without examination) 36 24 567 describes an analyzing system for the measuring of NO and SO.sub.2 which includes a deuterium lamp as the ultraviolet light source, a spectrometer and a photodiode array as a detector.
The periodical American Laboratory, (Volume 20, Issue 11, 1988, pages 88-91) describes a similar system which utilizes the same measuring principle.
These types of process measuring apparatus are utilized, for example, in power plants for monitoring the emission of ammonia, sulfur dioxide and nitrogen oxides. These known systems, however, function satisfactorily only in a limited range of concentration. Thus, for example, it is known that at high SO.sub.2 concentrations of approximately 1500 ppm, a strong background absorption in the ultraviolet range results, adversely affecting the measuring accuracy.
German Patent No. 27 27 976 discloses an apparatus for measuring at least one component of a gas mixture. The auxiliary cuvette of the apparatus contains a component B to be measured, rather than an interfering component A. The auxiliary cuvette is situated either continuously in the reference beam or is periodically pivoted thereinto.
The concentration of the component B is calculated from the difference between the measuring beam and the reference beam intensities or from the consecutive intensity values of the recorded measuring and reference beams.